Large quantities of paraffins are consumed in commercial processes which produce motor fuel alkylate by alkylation reactions and produce ethers including methyl tertiary butyl ether (MTBE) by etherification reactions. The production of these compounds is often a multistep process which requires recycling various intermediate paraffin-containing process streams. For instance a large etherification complex will normally contain a catalytic paraffin dehydrogenation unit and a catalytic isomerization unit in addition to the etherification unit. The incomplete conversion in these zones requires recycling paraffins recovered from the effluent of the etherification zone to the isomerization and dehydrogenation zones. This recycle stream will contain small amounts of nonparaffinic materials such as water or oxygenates including the product ether and feed alcohol. These nonparaffinic compounds are often detrimental to the catalysts and adsorbents employed in the next processing unit and it is necessary to remove them by some means.
The fresh feed stream to a process unit or to an overall processing complex may also contain compounds which are contaminants in the sense that they are injurious to the catalyst used in the process. In the case of a paraffinic fresh feed stream produced in a refinery these are likely to be indigenous sulfur compounds such as mercaptans.
1. Field of the Invention
The invention relates to a process for removing undesired contaminants from paraffinic feed streams destined for hydrocarbon conversion process units. More specifically the invention relates to a process for removing mercaptans, oxygenates and olefins from a paraffin rich stream through the use of catalytic distillation. The invention is directly concerned with a process to deliver pure C.sub.4 paraffins to a stand-alone isomerization unit or to an isomerization zone used in an etherification complex and which eliminates butenes, alcohol, water and mercaptans from the feed stream to the isomerization zone.
2. Related Art
An overall process flow for an etherification complex receiving an external feed stream and having a recycle stream from the etherification unit is shown in U.S. Pat. No. 4,465,870 issued to L. E. Herskovits. This reference employs adsorption to remove the small amounts of oxygenates such as the product ether and feed alcohol present in the C.sub.4 recycle or "raffinate" stream recovered from the effluent of the etherification zone to avoid deleterious impacts on the catalysts in the isomerization and/or dehydrogenation reaction zones of the overall complex. This reference also shows the admixture of the fresh butane feed stream, which is the potential source of mercaptans and olefins, into the recycled raffinate.
U.S. Pat. No. 4,775,462 issued to T. Imai and J. C. Bricker describes a non-oxidative method for the sweetening of a sour hydrocarbon fraction. The hydrocarbon fraction is contacted with an acid-type catalyst in the presence of an unsaturated hydrocarbon to convert the mercaptans to thioethers. The applicable catalysts are described as including polymeric sulfonic acid resins and butenes are indicated as suitable unsaturated hydrocarbons for the sweetening of LPG streams.
A paper entitled Reduce the Cost of Producing TAME by K. L. Rock presented at the 1994 National Petroleum Refiners Association annual meeting describes two treatment methods for the olefinic hydrocarbon feed stream which is to be charged to an etherification zone. One such method is to contact the feedstream with an aqueous caustic solution and to the convert the extracted mercaptans to disulfides via an oxidative reaction. The second method, described on page 4, involves the reaction of the reaction of diolefins present in the feed stream with the mercaptans which are also present in the feed stream to form a sulfide. This is performed in the presence of hydrogen used for olefin saturation.